Docs: Fix links to custom parsers doc

[btmills]

Prerequisites checklist

• I have read the contributing guidelines.
• The team has reached consensus on the changes proposed in this pull request. If not, I understand that the evaluation process will begin with this pull request and won't be merged until the team has reached consensus.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

The documentation for custom parsers was extracted from the main
"Working with Plugins" documentationn to its own page in #10431.

Is there anything you'd like reviewers to focus on?

I grepped the documentation to make sure I found all of the links:

$ grep -r 'working-with-custom-parsers' docs
docs/user-guide/configuring.md:1. It must conform to the [parser interface](/docs/developer-guide/working-with-custom-parsers.md).
docs/developer-guide/README.md:## Section 6: [Working with Custom Parsers](working-with-custom-parsers.md)
docs/developer-guide/nodejs-api.md:with the name of the parser as first argument and the [parser object](/docs/developer-guide/working-with-custom-parsers.md) as second argument. The default `"espree"` parser will already be loaded for every `Linter` instance.